The present invention relates to an access control system, and more particularly, to a control system for controlling the velocity of a head assembly in a direct access storage device.
A number of systems now exist and are commonly used in direct access storage device, such as a rotating magnetic disc, to control the velocity of a head assembly in moving from one location to the desired final location. In general, the two criteria that must be satisfied in such a system are: (1) the system must have fast response; and (2) the system must be accurate. Since substantially all of the present day direct access storage devices are used on digital computers, the command to the system from the computer is in digital form. Yet, on the other hand, the output from such a system to the head assembly must necessarily be in analogue measures, such as currents, and volts. Thus, present control systems must also deal with the problem of the conversion of digital information to analogue information. The problems of speed and accuracy are compounded by errors in the conversion. In IEEE Transactions on Magnetics, July 1978, Volume 14, No. 4, page 177, R. D. Oswald discusses the problems of speed and accuracy, and in particular the problem of accurately approaching the final desired location. In that article, Oswald also proposes a solution, wherein as the head assembly approaches the final desired position, a second access control subsystem is used to more accurately control the final phase of the arrival of the head assembly to the desired position. Such a system however, requiring more components, is clearly more expensive.